Gap junctions among the perikarya, dendrites, and axon terminals of the luminosity‐type horizontal cell of the turtle retina

Gap junctions of the H1 horizontal cell of the turtle retina (Leeper, 1978) were studied in thin‐sectioned material and in freeze‐fracture replicas. Peri‐karyal gap junctions were extremely restricted, 0.02–0.07 μm 2 in area, whereas those of axon terminals were much larger, most being 0.1–1.0 μm 2 . Both varieties, however, had the usual seven‐layered appearance in thin sec‐tion and measured 15 ± 1 nm in overall width between cytoplasmic faces. Freeze‐fractured views of the perikaryal junctions revealed roughly circular patches of P‐face 9‐nm particles and E‐face pits. The axon terminal gap junctions were seen as large areas of P‐face particles and E‐face pits containing occasional islands of unsnecialized membrane. Particle densities varied from 1, 455to 2, 448 μm −2 serial reconstruction was made or a portion of the axon terminal network in order to measure the surface areas of the axons con‐tained therein and the fraction occupied by gap junctions. These data demonstrated that the fractional area occupied by gap junctions was roughly in inverse proportion to the area of the axon region (tuberous core vs. terminal process). It is argued that this constitutes an impedance matching device to ensure adequate current flow through the axon processes. Assuming that each P‐face particle represents a connexon having a conductance of 10 −10 and given the P‐face particle density and gap junctional areas determined in this report, we calculated that the gap junction distribution is adequate to account for the spatial properties of the horizontal cell axon network (Lamb, 1976).